1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary battery in which a graphite material is applied as a positive electrode, a material capable of occlusion/desorption of a lithium ion, an alloy thereof or lithium is applied as an negative electrode, and a non-aqueous electrolyte containing lithium salt is applied as an electrolyte, and also relates to a method for preparing a positive electrode used for the non-aqueous electrolyte secondary battery.
2. Description of the Related Art
Conventionally, various non-aqueous electrolyte secondary batteries have been used in a wide range of applications because they have storable high energy density. However, they have shortcomings that at the time they reach a certain charge/discharge cycle, it becomes difficult to use them continuously, or it becomes impossible to use them anymore. For the purpose of improving a charge/discharge cycle life of these types of secondary batteries, the inventors of the present invention have focused on a non-aqueous electrolyte secondary battery which comprises a positive electrode formed of a graphite material, a non-aqueous electrolyte containing lithium salt, and an negative electrode formed of a material capable of absorption/desorption of a lithium metal or lithium, and they have diligently conducted research on such batteries.
There has been known a non-aqueous electrolyte secondary battery which comprises a positive electrode formed of a graphite material, an electrolyte containing lithium salt, and an negative electrode formed of a lithium metal. In addition, attempts have been made to enhance cycle characteristics by applying, as the negative electrode of the battery, a carbon material capable of absorption/desorption of lithium. See, for example, Japanese Patent Application Laid-Open Publication No. 61-7567, and Japanese Patent Application Laid-Open Publication No. 02-82466. This is because of a short cycle life of lithium metals, due to generation of dendrites and passivation of lithium metals resulting from repeated dissolution/deposition in accordance with charge/discharge cycles.
A non-aqueous electrolyte secondary battery with a configuration as described above is generally assembled to be in a discharged state, and the battery does not go into a dischargeable state unless it is charged. Hereinbelow, a case where a graphite material capable of reversible absorption/desorption of lithium is used as a negative electrode is taken as an example, and the charge/discharge reaction will be described.
Firstly, when charging at the first charge cycle, anions and cations (lithium ions) in an electrolyte are absorbed (intercalated) into the positive electrode (a graphite material) and the negative electrode, respectively. At the positive electrode, acceptor graphite intercalation compounds are formed, and at the negative electrode, donor graphite intercalation compounds are formed, respectively. Subsequently, upon discharging, cations and anions absorbed in the electrodes are desorbed (deintercalated), and the battery voltage is reduced. This charge/discharge reaction is represented by the following formulae.Positive electrode:(discharge) Cx+A−=CxA+e− (charge)Negative electrode:(discharge) Cy+Li++e−=LiCy(charge)
Accordingly, it can be said that a positive electrode used in this type of the secondary battery utilizes a reaction by which acceptor graphite intercalation compounds derived from anions in an electrolyte are reversibly formed along with charging/discharging.
Materials as a positive electrode that have been studied include: graphitized carbon fiber (see Japanese Patent Application Laid-Open Publication No. 61-10882, for example); expanded graphite sheet (Japanese Patent Application Laid-Open Publication No. 63-194319); a woven textile made of graphitized carbon fiber (Japanese Patent Application Laid-Open Publication No. 04-366554); plastic-reinforcing graphite; natural graphite powder; pyrolytic graphite; graphitized vapor growth carbon fiber; and PAN carbon fiber.
However, this type of battery has a shortcoming in that the discharge capacity is reduced when every charge/discharge cycle is repeated. The main cause of this is deterioration of a positive electrode material. Specifically, since anions having relatively large molecular size are repeatedly absorbed in, and desorbed from a graphite material along with the repeated charge/discharge cycles, thereby causing collapse of a graphite crystal and cracks in particles. As a result, a part of the graphite material changes into a form that does not allow charging/discharging.
On the other hand, there is an example that a battery using graphitized vapor growth carbon fiber showed a cycle life of 400 cycles or more by limiting the charge capacity as low as 36 C/g (=10 mAh/g), per unit weight of the graphite material for the positive electrode) and by charging/discharging the battery. However, further improvement in cycle life was required.
Note that, in this application, the term “graphitization” refers to transition from an amorphous carbon to a graphite which is caused by thermal energy, and specifically, it refers to heat treatment of the amorphous carbon at 1700° C. or above regardless of crystallinity after graphitization (see “Glossary of Carbon Terms (Kaabon Yougo Jiten)”, p. 114, 2000, Agune-shoufuu-sha). In addition, the term “carbon material” refers to a solid substance which contains a carbon atom as a principal component, in which the regularity of the carbon atoms is not specified. Similarly, the term “graphite powder” refers to a solid substance which contains a carbon atom as a principal component and has a crystal structure in which the carbon atoms are arranged with three dimensional regularity, and it does not matter whether or not it is a material that has been subjected to graphitization. Moreover, the average particle diameter is defined to be in the range of about 1 to 100 μm, as a general range.